The present invention relates to a laser diode device which is formed by sealing a laser diode chip with resin, and more particularly to an improvement of the laser diode device so that its sealing resin is prevented from being damaged by the laser beam from the laser diode chip, and the far field pattern thereof is satisfactory at all times.
A conventional laser diode device is, in general, of a can type as shown in FIG. 9. That is, a laser diode chip 10 is soldered to a radiator 62 mounted on a stem 61, and a cap 63 with a glass window is welded to the stem to seal the laser diode chip 10.
On the other hand, resih type light emitting elements have been employed in the light emitting elements low in optical density per unitary area such as light emitting diodes (LED), and resin-sealed laser diodes also have been known in order to make the laser diodes low in manufacturing cost and shaped with greater freedom. However, in sealing a light emitting element such as a laser diode high in optical density with resin, a problem is still involved which has not sufficiently solved yet. The problem is that the sealing resin is damaged by the laser beam, and the resultant light emitting element is therefore low in reliability or short in life time.
For instance, when a life test was given to a resin-sealed laser diode, which was sealed with a transparent epoxy resin high in optical transmission, under automatic power control (APC) for an ambient temperature of 60xc2x0 C. and an optical output of 3 mW, the sealing resin in contact with the laser beam emergence section (about 5 xcexcmxc3x971 xcexcm) of the laser diode was optically damaged within 100 hours; that is, holes were formed in the portions of the sealing resin which corresponded in position to the laser beam emergence section, thus lowering the characteristics of the laser diode.
On the other hand, it has been found that the above-described deterioration in characteristic of the laser diode can be effectively prevented by formation of an end-face breakage preventing layer between the laser diode chip and the sealing resin by using a material which is low in absorption coefficient with the band of wavelengths of the laser beam and high in heat resistance. The method has been filed with the Patent Office (U.S. Ser. No. 07/788,601 assigned to the same assignee as this application). The material of the end-face breakage preventing layer may be non-organic materials such as alumina, silica and low melting point glass, or organic materials such as silicone resin. For more information, see Japanese Patent Unexamined Publication No. Hei. 5-3377.
One example of the above-described resin-sealed laser diode device is as shown in FIG. 1, a perspective view. FIG. 2 is also a perspective view, with parts cut away, of a laser diode chip 10 shown in FIG. 1. FIG. 3 is a sectional view of the laser diode chip, taken along the longitudinal axis of an electrode 7.
The laser diode chip 10 is formed as follows: An n-type clad layer 3 of AlGaAs, an active layer 4, a p-type cladding layer 5, and p-type cap layer 6 of GaAs are formed on an n-type substrate 2 of GaAs in the state order. An electrode 7 is formed on the p-type cap layer 6, and another electrode 8 is formed on the substrate 2. In order to allow current to concentrate at the center of the active layer 4, the p-type cladding layer 5 has a current narrowing portion (not shown). The surfaces of light emitting end-faces 9 are coated with an insulating dielectric thin film, namely, an end-face protecting film 20b to a thickness of 0.5 xcexcm or less. In the chip 10, the light-emitting end faces have a laser beam emergence section about 5 xcexcmxc3x971 xcexcm. In addition, end-face breakage preventing layers 20, which are low in absorption coefficient to the band of wavelengths of the laser beam and high in heat resistance, are formed on the end-face protecting film 20b which is formed on the light emitting end faces 9 including the laser beam emergence sections. As shown in FIG. 4, a sectional view of a resin-sealed laser diode different in external configuration from the one shown in FIG. 1, the laser diode chip 10 with the end-face breakage preventing layers 20 is mounted on a heat radiating board 71 supported by a lead frame 72, and then sealed with a sealing resin 30 such as transparent epoxy resin so that the resin-sealed laser diode is formed.
The heat radiating board 71 is made of a Si substrate. A photo-diode 73 (see FIG. 1) is formed on a part of the upper surface of the heat radiating board 71. Its light receiving surface is in parallel with the above-described layers 2 through 6 and electrodes 7 and 8 of the chip 10, thus being able to monitor an emergent laser beam 101b (see FIG. 4) from the back side of the chip 10. The laser beam 101 emergent from the emergence section about 5 xcexcmxc3x971 xcexcm forms a divergent angle; i.e., a horizontal angle of about 10xc2x0 and a vertical angle of about 40xc2x0 in the half width of the laser beam intensity distribution. Accordingly, the beam area 102 (see FIG. 2) increases substantially in proportion to the square of the distance m which the laser beam 101 travels, whereas the optical density decreases substantially in proportion to the square of the distance m.
On the other hand, in the case where the light emitting end face of a laser diode chip high in optical density is covered with the end-face breakage preventing layer, as shown in FIG. 5 (which is an enlarged sectional view of the laser beam emergence section of the laser diode chip), the optical density of the laser beam 101 advancing towards the surface 20a of the end-face breakage preventing layer from the light emitting end face 9 is decreased at the surface 20a in proportion to the square of the thickness of the end-face breakage preventing layer 20.
Intensive research has been conducted with the above-described facts taken into consideration. During the research, it has been found that the thickness of the end-face breakage preventing layer 20 necessary for sufficiently reducing the optical intensity of the laser beam, and the life time of the resin-sealed laser diode are closely related to each other. This end-face breakage preventing layer, being greatly related to the present invention, will be described in more detail.
The material most suitable for formation of the end-face breakage preventing layer is silicone resin.
FIGS. 6(a) to 6(e) show a process of manufacturing a laser diode whose end-face breakage preventing layer is formed by using the silicone resin. First, a laser diode chip 10 having end-face protecting films 20b of Al2O3 on the light-emitting end faces is soldered, as indicated at 74, to a heat radiating board 71 of Si by a junction down method in such a manner that the chip 10 is adjacent to the light receiving surface 73a of a photo-diode 73 formed on the heat radiating board 71, and the light emitting end face of the chip 10 is flush with the side end face of the heat radiating board 71 (FIG. 6(a)). Next, the heat radiating board 71, on which the chip 10 has been fixedly mounted, is fixedly mounted in place on a lead frame 72 with an Ag epoxy adhesive 75 (FIG. 6(b)). Under this condition, the chip 10, the photo-diode 73, and external lead electrode terminals 721 and 722 of the lead frame 71 are connected by wire bonding (FIG. 6(c)). Thereafter, a dispenser (not shown) is used to apply a suitable amount of silicone resin to the chip 10 from obliquely above. The silicone resin thus applied is heated and hardened to form an end-face breakage preventing layer 20 (FIG. 6(d)). The resultant product is sealed with an epoxy resin 30, which transmits laser beam, to a predetermined configuration (FIG. 6(e)). Thus, the aimed laser diode has been manufactured. The laser diode is high in the flexibility of configuration. It may have the same configuration as the can type laser diode shown in FIG. 4. The laser diode shown in FIG. 6(e) is of a flat type which is high in productivity and in applicability to a variety of devices.
A laser diode having an end-face breakage preventing layer of dimethyl polysiloxane as the above-described one 20 was formed, and the relationships between its thickness and the life time were investigated. As a result of the investigation, the life time was ranged from 3800 hours to 12000 hours with a thickness of 20 to 30 xcexcm. The term xe2x80x9clife timexe2x80x9d as used herein is intended to mean the time which elapses until the sealing resin is optically damaged, so that the optical output of the laser diode is changed. Owing to the provision of the end-face breakage preventing layer, the life time of the resin-sealed laser diode is markedly increased, and its performance may be equivalent to that of the can type laser diode.
In addition, it has been found that, in 20 to 30% of the light emitting elements, the monitor current (Im) of the photo-diode adapted to receive the emergent laser beam at the back of the laser diode is greatly increased when the latter provides a predetermined optical output. The monitor current (Im) is used for controlling the laser beam emitted forwardly from the laser diode, and therefore it should not be greatly varied. Thus, those elements are determined unacceptable. Research has been conducted on the reason why the unacceptable elements having such large variation are formed, and found the following fact: As shown in a sectional view of FIG. 7, interfacial separations 80a are always formed between the silicone resin 20 on the photo-diode 73 formed in the heat radiating board 71 and the sealing resin 30 because of an interfacial stress which is caused by the difference in thermal expansion coefficient between them. And a part of the laser beam 101 applied to the interfacial separation 80a is reflected thereby as indicated at 101c, so that, in this case, the amount of laser beam applied to the photo-diode 73 is larger as much when compared with the case where no interfacial separation 80b is formed.
In order to promote the adhesion of the silicone resin, which is the material of the end-face breakage preventing layer, to the sealing resin thereby to prevent the separation of those resins as much as possible, a method has been proposed in which high-energy rays such as ultra-violet rays are applied to the surface of the silicone resin (see Japanese Patent Unexamined Publication No. Hei. 5-160521). The method is considerably effective in preventing the formation of the interfacial separations on the side of the front light-emitting end face, but on the side of the rear light-emitting end face it is not so effective, and the aforementioned unsatisfactory change in monitor current (Im) is not decreased so much.
The interfacial separation may be eliminated by making the thermal expansion coefficient of the end-face breakage preventing layer equal to that of the sealing resin or by using the same material for the end-face breakage preventing layer and the sealing resin. However, no appropriate combinations of those resins have been found yet.
In addition, in view of the present manufacturing technique, it is considerably difficult to form the end-face breakage preventing layers of silicone resin of a desired film thickness to cover only the front and rear light-emitting end faces.
The laser diodes subjected to the aforementioned life test were examined for electrical and optical characteristics during the test. As a result, it was found that about 1% of the laser diodes were unacceptable because the far field pattern (FFP) of the emergent laser beam was greatly irregular. In addition, a cyclical heat test was given to the laser diodes, one cycle comprising (1) a step of allowing the laser diode to stand at 85xc2x0 C. for thirty minutes, (2) a step of quickly cooling the laser diode to xe2x88x9240xc2x0 C. and allowing it to stand for thirty minutes, and (3) a step of quickly heating the laser diode to 85xc2x0 C. As a result, it was found that, when the number of cycles reached 200, the aforementioned unwanted phenomenon occurred frequently.
The cause for the occurrence of the unwanted phenomenon was investigated, and the following fact was found: As shown in FIGS. 16(a) and 16(b), separations 50 were formed at the interface between the end-face protecting film 20b of insulating dielectric such as Al2O3 coated on the light emitting end face, and a rubber-like organic silicone resin which is the material of the end-face breakage preventing layer 20. In the unacceptable light emitting elements, at least some of the separations 50 were located over the laser beam emergence section 51. On the other hand, in the acceptable light emitting elements, the separations were formed in the same manner; however, they were not located over the laser beam emergence section 51.
That is, it can be considered that the laser beam was scattered when passing through the separation formed between the end-face protecting film 20b of insulating dielectric and the end-face breakage preventing layer 20 of rubber-like organic silicone resin at the laser beam passing portion of the laser beam emergence section 51, thus making the far field pattern (FFP) irregular. The formation of the separations is due to the difference in thermal expansion coefficient between the laser diode chip 10 and the rubber-like silicone resin 20. This difference causes the end-face protecting film 20b and the end-face breakage preventing layer 20 to separate from each other.
In order to promote the adhesion of the end-face protecting film 20b to the end-face breakage preventing layer 20 thereby to prevent the separation of the film 20b and the layer 20 from each other, a method has been proposed in which high-energy rays such as ultra-violet rays are applied to the surface of the end-face protecting film 20b, and then rubber-like silicone resin is applied to the film thus treated. However, it is difficult for the method to completely solve the above-described problem.
In view of the above described problems, an object of the present invention is to provide a resin-sealed laser diode device in which separation between the end-face protecting film and the end-face breakage preventing layer of the laser diode chip is prevented, and separation between the end-face breakage preventing layer and the sealing resin is prevented, so that the far field pattern is not made irregular, the laser beam monitoring photo-diode is not varied in monitor current (Im), and the diode has a long lifetime.
The foregoing object of the present invention has been achieved by the provision of a resin-sealed laser diode device comprising: a laser diode chip having front and rear light-emitting end faces through which a laser beam from an active layer are emitted forwardly and backwardly; a lead frame supporting the chip through a supporting substrate; a sealing resin sealingly isolating the chip from the outside air; an end-face breakage preventing film of organic silicone resin low in the absorption coefficient to the band of wavelengths of the laser beam, for preventing the sealing resin near the light-emitting end faces from being damaged by the laser beam; and end-face protecting films made of an insulating dielectric coating the light-emitting end faces; wherein the end-face protecting films essentially contain silicon dioxide in its contact surface with the organic silicone resin, the organic silicone resin essentially contains rubber-like dimethyl polysiloxane, the rubber-like-organic silicone resin is a thermosetting resin, the end-face breakage preventing layer has a thickness of at least 50 xcexcm on the side of the front light-emitting end face on the extension of the surface of the active layer of the chip, the supporting substrate of the laser diode chip has the light receiving surface of a photo-diode which is in parallel with the surface of the active layer of the laser diode chip and monitors a laser beam emitted through the rear light-emitting end face, the laser diode chip is fixedly mounted on the lead frame with the front light-emitting end face held flush with the end face of the supporting substrate, the laser diode chip is fixedly mounted on the lead frame being shifted inwardly from the end of the lead frame to the extent that a laser beam emitted through the front light-emitting end face of the chip is not substantially blocked by the lead frame, and the position of the laser diode chip fixedly mounted on the lead frame is defined by the following expression: 0 less than X less than L cot(xcex8v/2) where X is the distance between the laser diode chip and the end of the lead frame, L is the distance between the lead frame and the active layer, and xcex8v is the full width angle at half maximum, in the vertical emission direction, of the laser beam.
In the device of the present invention, the surface of the end-face protecting film is coated with silicon dioxide to increase the strength of adhesion of the end-face protecting film to the end-face breakage preventing layer, thus preventing the interfacial separation. This eliminates the difficulty that the laser beam is scattered by the separations formed in the interface between the end-face protecting film and the end-face breakage preventing layer, thus making the far field pattern unacceptable. Furthermore, in the device, the end-face breakage preventing layer is formed by using thermosetting rubber-like silicone resin with a thickness of at least 50 xcexcm on the side of the front light-emitting end-face of the laser diode chip. Hence, the difference in thermal expansion coefficient between the end-face breakage preventing layer and the sealing resin can be absorbed by the expansion and contraction of the end-face breakage preventing layer, which prevents the occurrence of interfacial separation between the end-face breakage preventing layer and the sealing resin. Moreover, in the device, the end-face breakage preventing layer is large in thickness, and the optical damage to the sealing resin is therefore decreased as much, which increases the service life of the device. The laser diode chip has a stress-relieving structure which is provided by making the end-face breakage preventing layer large in thickness. In addition, the silicone resin layer on the side of the photo-diode is so shaped that its surface is curved upwardly with respect to the light receiving surface of the photo-diode. As a result, the stress induced in the interface between the end-face breakage preventing layer and the sealing resin by the difference in thermal expansion coefficient between them is concentrated into a region through which the laser beam does not pass. In other words, even in the case where the interfacial separation occurs partially because the stress is too large to be absorbed by the expansion and contraction of the rubber-like silicone resin, the interfacial separation can be limited in position so that it may not affect the electrical and optical characteristics of the laser diode.
In summary, by increasing the thickness of the end-face breakage preventing layer of rubber-like silicone resin, a resin-sealed laser diode device high in reliability with its electrical and optical characteristics maintained unchanged for a long time can be realized.